Probing the enigmatic Fermi surface of SmB6 by low-energy electrodynamics

在相关电子物理领域，六硼化钐(SmB6)是研究最多但也是最令人困惑的材料之一。传统上被认为是近藤绝缘体的SmB6最近被解释为第一个具有电子关联的拓扑绝缘体。这一概念要求由拓扑表面态的电荷引起的二维费米面，但最近关于SmB6的高调量子振荡结果严重冲突：一项研究报告了二维费米面的签名，而另一项研究则指出了三维费米面。后一种结果不能与SmB6在低温下真正绝缘的结果相一致，它引发了过多的新理论概念，其中一些引用了与固体中规则电子截然不同的奇异准粒子。在这个项目中，我们想要用低能电动力学的方法来阐明SmB6费米面的性质。特别是，我们想要检测和表征SmB6的回旋共振。这将是对费米表面性质的独立实验访问，无论后者是二维还是三维的。在常规固体中，回旋共振是通过在静磁场中对样品施加微波电场来探测的。对于SmB6的情况，相关移动准粒子的有效质量和散射率的值没有达成共识，甚至不清楚它们是电荷载流子还是中性粒子。因此，在这个阶段，我们无法预测在什么磁场和激发频率的组合下，在什么温度下才能观察到SmB6的回旋共振。因此，我们将使用各种实验技术(频率在45 MHz到1.3太赫兹之间，磁场高达8T，温度低至20MK，并可选择使用电场或磁场来选择性激发)来解开SmB6的电动力学响应之谜。这些在极宽参数范围内的实验将通过对回旋共振的探测来揭示费米面上的关键信息，此外，它们还将帮助我们解答SmB6性质中的另一个谜题，即先前报道的SmB6的太赫兹电导率是否由可能产生SmB6三维费米面的神秘准粒子引起，该电导率远高于已确定的直流电导率的预期。

In the field of correlated electron physics, samarium hexaboride (SmB6) is one of the most studied and yet most puzzling materials. Traditionally considered a Kondo insulator, SmB6 was more recently interpreted as the first topological insulator with electronic correlations. This notion calls for a two-dimensional Fermi surface caused by the charges of the topological surface states, but recent high-profile quantum-oscillation results on SmB6 are gravely conflicting: one study reported signatures for a two-dimensional Fermi surface whereas another indicated a three-dimensional Fermi surface. The latter result cannot be reconciled with SmB6 being truly insulating in the bulk at low temperature, and it has sparked a plethora of new theoretical concepts, some of them invoking exotic quasiparticles quite different from regular electrons in solids.In this project we want to elucidate the nature of the Fermi surface of SmB6 by means of low-energy electrodynamics. In particular, we want to detect and characterize the cyclotron resonance of SmB6. This will be an independent experimental access to the properties of the Fermi surface, regardless of whether the latter is two- or three-dimensional. In conventional solids, cyclotron resonance is probed by applying a microwave electric field to a sample in a static magnetic field. For the case of SmB6, there is no consensus on the values of effective mass and scattering rate of the relevant mobile quasiparticles, and it is not even clear whether they are charge carriers or neutral. Therefore, at this stage, we cannot predict at which combination of magnetic field and excitation frequency and below which temperature the cyclotron resonance of SmB6 will be observable. Hence we will use a variety of experimental techniques (at frequencies between 45 MHz and 1.3 THz, at magnetic fields up to 8 T, at temperatures down to 20 mK, and with option to selectively excite with either electric or magnetic high-frequency field) to unriddle the electrodynamic response of SmB6. These experiments in extremely wide parameter ranges will reveal key information on the Fermi surface via detection of cyclotron resonance, and in addition they will help us to answer another puzzle in the properties of SmB6, namely whether the previously reported THz conductivity of SmB6, which is much higher than expected from the established dc conductivity, is caused by the same enigmatic quasiparticles that possibly generate the three-dimensional Fermi surface of SmB6.